The invention is related to a technology for measuring a water level using a sonic wave, and particularly, to a method for exactly measuring a water level using a sonic wave independent of a measuring range in a reservoir, a underground water and a larger river, etc., and system therefore.
A measuring range of a reservoir, a river and a underground water may be larger, but the hydrology observation requires not to exceed the allowance errorxc2x151xcx9c10 mm throughout a full range of a water level to be measured.
Typical sonic level measuring methods and/or systems therefore for satisfying such conditions are disclosed as follows:
U.S. Pat. No. 5,842,374 issued on Dec. 1, 1998
Germany Patent No. 19511234 published on Sep. 11, 1997
Japanese Patent No. 2,756,647 issued on Mar. 13, 1998
Korean Patent No. 150,714 issued on Jun. 16, 1998
A conventional sonic water level measuring method will be described with reference to FIG. 1. 1 is a sound generator, 2 is a wave-guide tube and 51, 52, 53 . . . 5n are a sound receiver.
The first sound receiver 51 is placed on an original position 0, and a distance L to a water surface therefrom is measured.
The sound generator 1 is operated to transit a sonic pulse toward a water surface along the wage-guide tube 2. Then, the sonic pulse is reflected on the water surface and transited upward. At that time, a time interval t1 from a moment that the sound receiver 51 receives an incident wave until it receives the sonic pulse reflected on the water surface is as follows: (the measuring error of t1 is ignored.)                               t          1                =                              2            ⁢            L                                C            1                                              (        1        )            
Wherein, C1 is a sonic velocity in an interval L.
Similarly, a time interval t2 from a moment that the sound receiver 51 receives an incident wave until the sound receiver 5n receives the incident wave transited thereto is as follows:                               t          2                =                                            L              0                                      C              2                                =                                                    (                                  h                  -                  1                                )                            ⁢              l                                      C              2                                                          (        2        )            
Wherein, C2 is a sonic velocity in an interval L0, n is the number of the sound receiver and l is an interval between the sound receivers 5i and 5i+1.
Therefore, L is obtained from the expressions (1) and (2) as follows:                               L          xe2x80x2                =                                                            t                1                                            2                ⁢                                  t                  2                                                      ·                          L              0                                =                                                    t                1                                            2                ⁢                                  t                  2                                                      ⁢                          (                              n                -                1                            )                        ⁢            l                                              (        3        )            
But, the exact value of L is as follows:                     L        =                                            t              1                                      2              ⁢                              t                1                                              ⁢                      L            0                    ⁢                                    C              2                                      C              1                                                          (        4        )            
Wherein, the expression (3) is established under the assumption that C1=C2.
In summer, an air temperature in the upper portion of the wave-guide tube is higher than that in the lower portion of the wave-guide tube. On the contrary, when the air temperature is lower than a water one, the air temperature in the upper portion of the wave-guide tube becomes lower than that in the lower portion of the wave-guide tube. Therefore, for C1=C2, L≈L0 must be established.
In other words, the interval l between the sound receivers is selected to become as small as possible, and an interval xcex94L=Lixe2x88x92L0i of the sound receiver disposed closest to the water surface in the wave-guide tube is selected. In the conventional sonic water level measuring system, the value l is selected as follows:                     l        ≤                              Δ            L                    ⁢                      xe2x80x83                    ⁢                                                    C                0                            +                              0.5                ⁢                                  α                  ⁡                                      (                                                                  T                        0                                            +                                              T                        w                                                              )                                                                                      0.5              ⁢                              α                ⁡                                  (                                                            T                      0                                        -                                          T                      w                                                        )                                                                                        (        5        )            
Wherein, T0 is an air temperature in the position of the sound receiver 51; TW is an air temperature on the water surface of the wave-guide tube; C0 is a sonic velocity of 331.6 m/s, when T=0xc2x0 C.; xcex1 is a temperature coefficient, in which xcex1≈0.6; and xcex94L is an allowance error of the L measurement.
The expression (5) is derived under the assumption that the air temperature is changed with a constant gradient (T0xe2x88x92TL)/L of a straight line (referring to FIG. 5).
When T0=40xc2x0 C., TW=25xc2x0 C. and xcex94L=0.01 m(1 cm), l is as follows:
lxe2x89xa60.78 m
If the water level is changed in the range of 20 m, the number of the receiver 5i is as follows:       n    ≥          20      0.78        =      25.6    ≈    26  
If the allowance error of the water level measurement xcex94L=xc2x15 mm, N=52. In other words, a large number of the sound receivers are required.
Conventional technical features are that the interval between the sound receivers gets narrowed, and the number of the sound receiver must be increased so that the accuracy of the water level measurement is enhanced
But, the disadvantages are as follows:

The increasing of the number n of the sound receiver causes the water level meter to become complex and thus results in heightening the failure possibility of the sound receiver. For example, the sound receivers are positioned below or over the water surface according to the water level change. If the water level is raised, the sound receiver disposed on the lowest portion of the system is under a larger water pressure. Against this situation, the sound receivers are thoroughly waterproofed and their receiving sensitivities must be kept at a uniform state in air as well as in water. And, the sound receivers must be made into a compact size free of the failure. Due to these reasons, the sound receiver gets complex and expensive. It is found out from much experience that the failure ratio of the sound receiver is highest among parts of the system.
A next problem exists in being not able to narrow the interval l between the sound receivers, auxiliary. The reason is as follows: In order to secure the higher water level measuring range, a sonic pulse of a lower frequency is used. As shown in FIG. 2, it takes much time   τ  =            6      1000        =          6      xc3x97              10                  -          3                    ⁢      S      
until the sonic pulse is fully attenuated. In FIG. 2, a dotted line is a reflected pulse. If f=1000 Hz, the reflected pulse requires 6 periods for the fully attenuation, in which the period.
A transit time txcex94L that the sonic pulse is reflected on the water surface and transited to the sound receiver closest to the water surface is as follows:       t          Δ      ⁢              xe2x80x83            ⁢      L        =            2      ⁢      Δ      ⁢              xe2x80x83            ⁢      L              C              Δ        ⁢                  xe2x80x83                ⁢        L            
In order to get the sound receiver to receive an incident wave and then reflected wave, exactly, txcex94Lxe2x89xa7xcfx84. If Cxcex94L=348 m/s, xcex94L is as follows:       Δ    ⁢          xe2x80x83        ⁢    L    =                    τ        ·                  C                      Δ            ⁢                          xe2x80x83                        ⁢            L                              2        =                            6          ·                      10                          -              3                                ·          348                2            =              1.04        ⁢                  xe2x80x83                ⁢        m            
It means that l can""t be selected below 1.04 m. In order to select a small value of l, the frequency of the sonic pulse must become larger. As the frequency of the sonic pulse is increased, the damping becomes larger. For it, the water level measurement can""t be secured in a larger range. The water level measurement range is usually 50 m.
The patent discloses that l is selected to be 0.78 m in order to secure the water level measuring error xcex94L=xc2x11 cm, but if the frequency f of the sonic pulse is equal to 1000 Hz, it is not possible to measure the water level. Even though l is selected to be 1.04 m, the corresponding sound receiver is switched into another sound receiver to be operated thereover, when the water level xcex94L rises a little. As a result, if l is selected to be larger than 1.04 m, the water level measuring error is increased. It is now used to be l≈2 m for the manufacturing of the system.
The conventional technology has limitations in securing the accuracy of the water level measurement. Also, the accuracy of the water level measurement can""t be secured, even though the number of the sound receiver is increased and l is decreased.
Accordingly, an object of the invention is to provide a sonic water level measuring method for securing the higher accuracy in a wider water level measuring range with a number of sound receivers being not used and system therefor.
A sonic water level measuring method and system therefor comprises two sound generator mounted toward the water surface on the upper of a wave-guide tube, which are spaced away at an interval l1 from each other; a float submerged in water and including a projector integrally extended therefrom with a diameter of 0.5 times as large as that of the wave-guide tube, the upper surface of which is kept at a height of l2; and a sound receiver receiving incident waves upon the generating of a sonic pulse and then sonic waves reflected in order on the upper surface of the projector and the water.
Therefore, a sonic water level measuring method comprises steps of measuring a sonic pulse transit time tL from a moment that a first sound receiver receives an accident wave until the first sound receiver receives a wave reflected on the water surface, measuring times t1 and t2 that the sonic pulse is transited in an interval l1 between two sound receivers and in an interval l2 between the upper surface of the projector and the water surface and measuring a water level based on the measured values as follows:                     L        =                                            (                                                t                  L                                2                            )                        ·                          1              2                                ⁢                      (                                                            l                  1                                                  t                  1                                            +                                                2                  ⁢                                      l                    2                                                                    t                  2                                                      )                                              (        6        )            
Wherein, two sound receivers disposed on the upper of the wave-guide tube is always mounted to be in air, and the projector integrated with the float is called xe2x80x9cSound Reflecting Cylinderxe2x80x9d below.